1. Field of the Invention
The present invention relates to a color image processing apparatus and, more particularly, to a color image processing apparatus that produces a print proof either by outputting a hard copy on a color printer or by displaying an image on a CRT, in either case taking account of halftone dots as well as variations in the operating conditions of a printing press.
2. Description of the Related Art
When producing color printing on a printing press, for example, a web rotary offset printing press, aluminum plates of four colors of CMYK (cyan, magenta, yellow, and black) must first be made in the platemaking step. Printing units for CMYK in a web rotary offset printing press each include a plate cylinder and a rubber cylinder, and the aluminum plate is wrapped around the plate cylinder. Water from a water reservoir is applied to the plate cylinder via water rollers while ink from an ink reservoir is applied to the plate cylinder via ink rollers, and printing is accomplished by transferring the applied ink to a page surface via a blanket wrapped around the rubber cylinder. Usually, in the platemaking step, CMYK plates are binarized by applying halftone dot processing to the multi-value images of CMYK created by reading color images from an original such as a picture or a painting. CMYK films are produced from these binarized plates, and the aluminum plates, that is, the printing plates, are produced from these films; the printing plates are then mounted on the printing press to produce the final printed product.
Many steps have to be performed, as described above, before the final printed product can be obtained, and the process consumes much time and labor. If the color of the final printed product differs from the desired color, the steps involved must be redone, wasting time and labor. Attempts have been made to avoid such wastage by producing a print proof at an early stage of the process. One method, for example, proposes thermally transferring ink to a page using the films. However, this method requires the use of films; besides, it is difficult to make adjustments taking account of dot gain and plate registering in the printing press, and a lot of time and labor is required even with skilled labor.
In view of this, there have recently been proposed color image processing apparatuses that make a printing proof without requiring films. Such color image processing apparatuses create, for example, 700 to 800 patches based on multi-level CMYK values for printing, produce printing plates by binarizing them, obtain first color values by measuring the final printed product obtained through the printing press process, and create first tables for converting the multi-level CMYK values to the first color values. The first color conversion using the first table is a conversion to a color space that does not depend on a device, such as a color printer or a CRT, used to produce an approximate output. Next, 700 to 800 patches, for example, are created based on the multi-level CMYK values to be input to a color printer used as a device for producing an approximate output, and second color values are obtained by producing an output on the color printer; then, a second table is created for converting the color values to the multi-level CMYK values for the color printer. The second color conversion using the second table is a conversion from the device-independent color space to a color space that depends on a device such as a color printer. In this way, the multi-level CMYK values for printing are converted to the multi-level CMYK values for the color printer through the first and second color conversions and output to the color printer to obtain the print proof output.
For the first color conversion, a method is proposed that performs the conversion using Yule-Nielsen or Neugebauer theoretical equations in order to reduce the number of color patches used.
However, with the proofing method according to the above prior art, multi-value images of CMYK have had to be input, and it has not been possible to produce a proof for binary data. Also, the prior art method has had the problem that colors cannot be reproduced with halftone dots since no account is taken of the halftone dots used in printing. Furthermore, patches have had to be output over and over again as the operating conditions of the printing press (printing speed, roller pressures, etc.) change.
It is, accordingly, an object of the present invention to provide a color image processing apparatus that permits an input of binary data and that is capable of outputting a proof taking account of the reproduction of halftone dots as well as variations in the operating conditions of a printing press.
A color image processing apparatus according to the present invention, which accomplishes the above object, accepts an input of halftoned binary image data and, using a dot gain processing unit, converts the binary data to multi-value data in such a manner as to simulate the amount of dot gain that changes with printing conditions. After conversion to the multi-value image data, the multi-value image data is converted by a device-independent color space converting unit (first color-converting unit) to image data in a device-independent color space. Next, the resolution of the image data is converted by a resolution converting unit to the resolution of an output device. Finally, the multi-value image data converted into the device-independent color space is converted by a device color space converting unit (second color-converting unit) to image data in the color space of the output device, and the thus converted image data is output.
With this configuration, since the dot gain effect is taken into account when performing the binary to multi-value data conversion, correct colors can be reproduced from the binary input, and halftone dots can also be reproduced.
In the above color image processing apparatus, the dot gain processing unit obtains the multi-value image data using a matrix defined by data of pixels surrounding each pixel in the binary image.
This configuration enhances proofing accuracy by capturing the thickening of ink, associated, for example, with a roller pressure in a printing machine, as the spreading of a pixel into its surrounding area.
In the above image processing apparatus, the size of the defined matrix and each row/column value in the matrix are determined by the resolution and the amount of dot gain of the binary image.
With this configuration, a matrix is automatically generated from the resolution and the amount of dot gain of the binary image that determine the matrix size and values.
In the above image processing apparatus, the size of the matrix and each row/column value in the matrix are determined using information such as the number of rows of halftone dots and halftone dot patterns.
With this configuration, more accurate matrix sizes and matrix values can be obtained.
In the above image processing apparatus, the size of the matrix and each row/column value in the matrix are determined independently for each color corresponding to each printing plate.
In the above image processing apparatus, the device-independent color space is a color space where additive color mixing holds.
With this configuration, changes in color are indiscernible to the human eye if the resolution is converted.
In the above image processing apparatus, the first color-converting unit which converts the multi-value image data into the device-independent color space uses a conversion based on Neugebauer theoretical equation.
With this configuration, since the binary to multi-value conversion is performed taking account of the dot gain effect, the dot gain effect need not be taken into account when converting the multi-value image data into the device-independent color space.
In the above image processing apparatus, after converting the multi-value image data, converted into the device-independent color space, to image data in the color space of the output device, the second color-converting unit converts the image data to binary image data using an error diffusion method and outputs the binary image data onto the output device.
This configuration serves to prevent the occurrence of moire resulting from the halftoned binary data.
The above image processing apparatus includes a gray adjusting unit for making gray adjustments by inputting the color of printing paper and the white color of the output device into the device-independent color space.
This configuration enables the printing paper to be reproduced as the white color of the output device.
The above image processing apparatus includes: a resolution adjusting unit for receiving an input of binary image data output at a plurality of resolutions and, after converting the binary image to a multi-value image for each resolution, adjusting the resolution of the multi-value image to the highest of the plurality of resolutions; and a color combining unit for combining the multi-value images on a color-by-color basis.
This configuration prevents degradation of image quality.
In the above image processing apparatus, the size of the defined matrix and each row/column value in the matrix are determined by sharpness.
With this configuration, fine adjustments can be made in the reproduction of halftone dots.
In the above image processing apparatus, the dot gain processing unit, which receives the input of the halftoned binary image data and converts the binary image data to multi-value image data corresponding to the amount of dot gain of each color, calculates data of each pixel by: entering all combinations of binary data of pixels surrounding the pixel; storing the combinations in the device-independent color space; performing dot gain processing and device-independent conversion processing; and creating a table from the results of the processing.
Using the table created by the above configuration, processing can be performed at high speed.
To achieve the aforementioned object, the invention also provides a recording medium readable by a computer and used in a color image processing apparatus for producing a print proof, the recording medium having a program recorded thereon for making the computer implement the functions of: a dot gain processing unit for receiving an input of halftoned binary image data, and for converting the binary image data to multi-value image data corresponding to the amount of dot gain of each color; a first color-converting unit for converting the multi-value image data to image data in a device-independent color space; a resolution converting unit for changing the resolution of the multi-value image in the device-independent color space to match the resolution of an output device; and a second color-converting unit for converting the multi-value image data, converted into the device-independent color space, to image data in a color space associated with the output device.